1. Field of the Invention
The present invention relates to a mass spectrometer and amass spectrometry method in which sample components separated by a liquid chromatograph are ionized, and the obtained ions are cleaved and subjected to mass spectrometry to perform MSn analysis (n is an integer of 3 or greater).
2. Description of the Related Art
Sample components may be identified based on a MSn spectrum obtained by separating sample components by a liquid chromatograph and sequentially subjecting the separated sample components to MSn analysis in identification of an organism-derived high-molecular compound such as a peptide. In this case, mass spectrometry (MS analysis) of an ionized sample component is first performed to measure a MS spectrum. The measured MS spectrum is then correlated with a retention time to obtain three-dimensional information of a retention time (RT), a mass-to-charge ratio (m/z) and an intensity (I).
FIG. 8 is a view showing one example of three-dimensional information of a retention time, a mass-to-charge ratio and an intensity obtained from MS analysis. In this example, peaks of mass-to-charge ratios mz1 and mz2 are detected in MS spectra of mutually adjacent retention times rt1 to rt3. The intensity of the mass-to-charge ratio mz2 is low at the retention time rt1, and the mass-to-charge ratio mz1 is low at the retention time rt3.
In this case, ions corresponding to the peak P11 of the mass-to-charge ratio mz1 for the retention time rt1, the peak P21 of the mass-to-charge ratio mz1 and the peak P22 of the mass-to-charge ratio mz2 for the retention time rt2, and the peak P32 of the mass-to-charge ratio mz2 for the retention time rt3 are detected as MS/MS precursor ions. Thereafter, at each retention time, the MS/MS precursor ion is cleaved and subjected to mass spectrometry (MS/MS analysis) to measure a MS/MS spectrum. Sample components can be identified by performing search using a known database based on the MS/MS spectra obtained as described above.
However, for example, when a peptide as a sample component is subjected to post-translation modification, or ions are not sufficiently cleaved in MS/MS analysis, sample components may not be identified by database search as described above. In this case, it is effective that after MS/MS analysis is performed, a MS3 precursor ion is further detected from the MS/MS spectrum, and the MS3 precursor ion is cleaved and subjected to mass spectrometry (MS3 analysis) to measure a MS3 spectrum (see JP-A-2012-251878).
When database search is performed based on a MS spectrum, a MS/MS spectrum and a MS3 spectrum that are obtained by performing analyses up to MS3 analysis, identification results can be obtained with higher reliability.
However, when MS3 precursor ions are detected from all MS/MS spectra obtained by MS/MS analysis and MS3 analysis is performed, there is the problem that the measurement time is prolonged and the consumption of a sample is increased.
The present invention has been devised in view of the above-described situations, and an object of the present invention is to provide a mass spectrometer and a mass spectrometry method which can realize shortening of the measurement time and reduction of the consumption of a sample.